Silicon based microelectronic drive circuits deposited on glass substrates are used extensively in the liquid crystal display. Presently, the silicon used is amorphous or near amorphous in structure. Consequently, the silicon exhibits carrier mobilities that are less than one centimeter squared per volt-second (1 cm.sup.2 /Vs). The problem arises because glass is an amorphous substrate. Hence, any silicon deposited on glass is either amorphous or polycrystalline. Instead, it is desirable to prepare a single crystal film of silicon on a glass substrate in an economical manner, since single crystal films have much higher carrier mobilities.
Polycrystalline silicon has low carrier mobilities because of a high density of grain boundaries that act as scattering centers. In the limiting case of very small grains, there is a consequent high degree of disorder. Single crystal, or near single crystal films, on the other hand, have either none or very few grain boundaries. Consequently, the carrier mobilities of single crystal or near single crystal silicon are very high.
There are two types of single crystal films. A first type has no grain boundaries and extended defects, such as dislocations. These films have the highest carrier mobilities and are usually found only for the case of homoepitaxy, or near homoepitaxy. The second type of single crystal films has no grain boundaries, but could possess domain boundaries and/or a high density of dislocations. These films can still have significantly high carrier mobilities, though not as high as the first type. For instance, Bean describes in Applied Physics Letters, volume no. 36, pages 741 to 743, 1980, a single crystal silicon grown on sapphire substrates, with a high lattice mismatch resulting in a very large density of dislocations. However, the carrier mobilities are on the order of 200 to 300 cm.sup.2 /Vs, which is two orders of magnitude higher than what can be expected from amorphous silicon.
Organic thin film-based microelectronics on flexible substrates, such as plastic, is an emerging area of research for applications such as smart cards or flexible displays. However, these materials also suffer from poor carrier mobilities of less than 1 cm.sup.2 /Vs. One advantage of such thin films is their ability to flex. However, if a single crystal silicon thin film can be cheaply grown or transplanted onto a plastic or polymer substrate, it will provide superior electrical properties compared to organic thin film based devices. Furthermore, silicon has excellent elastic properties so that a thin (less than a few microns) film will easily conform to a flexing substrate.
The problem essentially is how to form single crystal thin silicon films onto large area glass or plastic substrates in an economical fashion to create high mobility devices that provide a better alternative to amorphous silicon on glass or organic electronics on plastic.
Possible solutions are to either transfer or grow a high quality single crystal silicon layer onto a plastic or glass substrate. Growth of a single crystal film on an amorphous substrate is an unsolved challenge. There are different ways of transferring a silicon layer to an arbitrary substrate, most of which are uneconomical or impractical. One method is to diffusion bond a silicon wafer to the glass or plastic substrate, and then etch off the silicon from the backside to remove most of the substrate. However, this method destroys an entire silicon wafer such that a large area coating is therefore uneconomical. Another method is to perform the diffusion bonding and then, using a "smartcut" H embrittlement process, peel away silicon layers from a wafer. A single wafer is then able to supply a large amount of semiconductor real estate. However, a smartcut process requires a 600.degree. C. anneal, which is incompatible with commonly used glass or plastic substrates.
U.S. Pat. No. 5,225,251 describes a process for coating a substrate surface with aluminum by depositing a layer of aluminum nitride on the substrate surface. The process uses a metallization step whereby the aluminum nitride is irradiated with ultraviolet radiation to dissociate the nitrogen such that a coating of aluminum remains on the substrate surface.
An article by Wong and Sands, entitled "Damage-Free Separation of GaN Films From Sapphire Substrates", Applied Physics Letters, volume no. 72, pages 599 to 601, 1998, describes the use of a metallization step for lift off or transfer from a sapphire substrate. The article starts with a structure having a gallium nitride film deposited on a sapphire substrate. A silicon wafer is then bonded with epoxy to the gallium nitride film. A metallization step is then used to lift the sapphire substrate from the gallium nitride film/silicon wafer structure.
It is an object of the present invention to resolve the problem of cheaply fabricating a substantially single crystal semiconductor on a host substrate.
It is another object of the present invention to use a metallization procedure to transfer a semiconductor structure grown on a nitride film disposed on a sapphire substrate to the host substrate.